1. Field of the Invention
The present invention relates generally to a thick, high-strength steel casting having high mechanical strength and toughness and excellent weldability.
2. Description of the Prior Art
At present, there is extensive demand for thick, high-strength steel castings which have a tensile strength of 80 to 90 kg/mm.sup.2 for use as construction materials. These materials are used, for example, as leg nodes for oil rigs in boring oil wells at the sea bottom.
In order to increase the mechanical strength of a steel casting, it is known to add various alloying elements. However, the addition of alloying elements in great quantities not only increases the cost of the steel material but also increases the Ceq value (carbon equivalent)=[C+(1/6)Mn+(1/24)Si+(1/40)Ni+(1/5)Cr+(1/4)Mo+(1/14)V] which tends to deteriorate the weldability of the material. Since, in many cases, high strength steel casting products are assembled together with other members by welding and since in many cases, an after-treatment after the welding is impossible, deterioration of the weldability and toughness due to the increase of the aforesaid Ceq value is undesirable.
In general, it is well known to add boron into steel in order to improve the hardenability of the steel without greatly increasing the Ceq value thereof. Steel castings with tensile strength of 80 kg/mm.sup.2 or more provided by adding 0.005 to 0.006% boron are proposed in Japanese Patent Publication No. 36451/1973. However, the disclosed steel castings incorporate a great quantity of boron in order to provide high mechanical strength in its normalized state and therefore the impact properties of the casting are inferior.
While it is well known that the inclusion of boron generally improves the hardenability of steel as mentioned previously, the effect of boron does not appear by merely adding boron into the steel but is influenced by the presence of other components, particularly nitrogen and aluminum. Furthermore, the effect of boron is greatly influenced by the thermal hysteresis of the steel material before heat treatment.
Such behavior of boron in steel has been reported by the present inventors in "Iron and Steel", Vol. 62 (1976), pages 1,842 to 1,857, in "The Sumitomo Search", No. 15, May 1976, pages 27 to 41 and also in "ASME Conference paper 76-Pet-61". In these reports, the relationship between the heating temperature during rolling and hardenability is theoretically elucidated with regard to high-strength plates, i.e., rolled plates, containing boron.
Although the behavior of boron in steel plate manufactured by rolling is made clear by the aforementioned reports, there is no report regarding the effect of boron in cast steel which is not rolled, particularly as to the improvement in the mechanical strength and also to the improvement in toughness and weldability after quenching and tempering in such cast steel.